Generators generally have a moving rotor and a stationary stator to generate electrical energy. Either the rotor or the stator comprises a yoke with a permanent magnet. The other part comprises laminations with windings.
Magnet flux is an important parameter of a generator as it contributes to the torque of the generator. Leakages of magnet flux occur during operation of the permanent magnet machine and reduce the torque.
In the end regions of the generator i.e. at the axial ends of the generator magnet flux leakage occurs. It was realized to be significant especially for a direct drive generator in which high magnetic loading of the stator laminations is desired for high ratio of output power to weight. Due to high magnetic loading, the stator ferromagnetic material is magnetically saturated and this results in high value of flux leakage in the end regions of the permanent magnet where it is easier for the flux lines to close their path through the rotor and avoid linking the stator windings and thereby their contribution to torque decreases by getting closer to the end regions. Furthermore, in direct drive generators with large diameters and short axial length the ratio of the magnet flux leaking to the portion of the magnet flux that links the stator winding and contributes to the torque becomes more significant.
The situation described above does not apply solely to outer rotor direct drive generators but to all other types of permanent magnet machines as well. However, this situation is of importance especially for outer rotor direct drive generators.
In the literature it is proposed to shorten the length of the rotor compared to the stator. However, this method is only practical for inner rotor generators or motors where the rotor yoke underlying the magnets ends where the magnet ends, seen in an axial direction. However, external rotor generators are preferred to inner rotor generators due to their higher output power to weight ratio.